For the continuous production of PET, PTA or dimethyl terephthalate (DMT) and EG are used as starting substances. PTA is mixed with EG and a catalyst solution to form a paste and charged to a first reaction stage for esterification, in which esterification is effected at atmospheric or superatmospheric pressure by separating water. When DMT is used, the DMT melt and the catalyst together with the EG are supplied to a first reaction stage for transesterification, in which the reaction is effected at atmospheric pressure by separating methanol (MeOH). The product stream of the esterification/transesterification is supplied to a reaction stage for prepolycondensation, which generally is performed under a vacuum. The product stream from the prepolycondensation is introduced into a reaction stage for polycondensation. The polyester melt obtained is directly processed to obtain fibers or chips.
The conventional process for producing PET comprises two stirring stages each for esterification and prepolycondensation and a horizontal cascade reactor for polycondensation, which includes bottom chambers and a stirrer equipped with vertical perforated or ring disks on a horizontal shaft for the purpose of producing a defined surface. The disadvantages of this process must in particular be seen in that inside the cascade reactor comparatively high temperatures of 284 to 288° C., which are disadvantageous for the quality, occur with sufficiently large flow rates. The vacuum applied in the first stirring stage of the prepolycondensation to avoid foaming and entrainment of droplets is limited to p≧50 mbar. The viscosity of the prepolycondensation product likewise is limited to a range of 0.20 to 0.24 IV. What is furthermore disadvantageous is the increased gas yield in the cascade reactor forming the polycondensation stage. The use of a horizontal cascade reactor instead of the second stirring stage for prepolycondensation allows a high flexibility of the PET production with comparatively lower temperatures of 277 to 283° C. in the cascade reactor for polycondensation and an increased viscosity of the prepolycondensation product of 0.27 to 0.31 IV as well as optimum possibilities for increasing the plant capacity (Schumann, Heinz-Dieter: Polyester producing plants: principles and technology. Landsberg/Lech: Verlag Modeme Industrie, 1996, pp. 27 to 33). What remains disadvantageous, however, are the high investment costs for the apparatus involved and the company building.
In a plant comprising two stirred tanks for esterification, a multilevel reactor for prepolycondensation and a horizontal cascade reactor for polycondensation, a comparable stability and flexibility of the polyester production is obtained with relatively little effort, but with the disadvantage that the dimensions of the reactors of the prepolycondensation and polycondensation stages will be increased because of increased vapor volumes, and the admissible transport dimensions are reached already with mean plant capacities.
In the process of producing PET by means of four reaction stages, which is described in DE-C-4415220, a vertical reactor is used each for postesterification and prepolycondensation. In its initial upper region, the reactor has a helical duct with product inlet at the wall, which is open at the top and, extending from the outside to the inside, communicates via a central overflow with a stirred product sump disposed at the bottom, the duct bottom ascending continuously in flow direction, so that the depth of the product stream is decreasing continuously. Heating the product stream is effected by means of individual radiators initially repeated at intervals and optionally via the duct walls. By means of the duct bottom ascending in flow direction an automatic system evacuation in flow direction is prevented with the consequence of the formation of residues, quality deterioration or product losses, in particular in the case of operating troubles or when shutting down the production plant. Due to the evaporation surface limited by a single duct bottom, the operating vacuum is either restricted, the operating temperature is increased, the color quality of the product produced is decreased, or an increased vacuum involves the risk of an excessive vapor velocity and an entrainment of droplets critical for a trouble-free condensation; this effect is intensified in addition by locally concentrated radiators in the flow duct.
U.S. Pat. No. 5,464,590 discloses a vertical polymerization reactor with a plurality of trays disposed vertically one above the other, which each have two flow ducts open at the top at the respective duct end, and an overflow weir with adjoining bottom recess for the vertical transfer of product in the form of a freely falling film on the succeeding tray. The duct has the shape of an approximately ring-shaped double loop, the respective first loop semicircularly being deflected into the second opposed loop. The liquid polymer traverses the ducts from the top to the bottom in a free movement. The vapors each flow between the trays towards the middle of the reactor and escape via central tray apertures to the vapor outlet at the reactor lid. A low filling level on the trays and a heating restricted to the bottom region involve a dwell time deficit for the prepolycondensation of esterification products with restricted polymer degrees of 4.5 to 7.5 with the consequence that either the number of trays or the dimensions of the reactor must be increased. Another disadvantage must be seen in that reaction space is lost due to the incorporation of guides and free falling-film zones. Due to the horizontal arrangement of the duct bottoms and due to the overflow weirs, a residue-free continuous operation of the reactor and/or its complete drainability are not ensured.
The same disadvantages are also obtained with the polymerization reactor represented in U.S. Pat. No. 5,466,419, in which the product charged to the first ring duct is divided in two partial streams, which each traverse two semicircular loops and half a distance up to the flow reversal and the overflow weir or the product outlet, i.e. with a comparable duct cross-section the flow velocity likewise is halved and the hydrostatic pressure difference is reduced.